Page 191 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

168 High Temperature Solid Oxide FueI Cells: Fundamentals, Design and Applications

are two metals with a total valency of 6, form a wide variety of solid solutions by
partial substitution on either the A or B sites, or both. These compounds are
established in SOFC technology with the chromites used as electrical
interconnects and gas separators, for example. They have the required stability
over the whole range of fuel cell oxygen partial pressure from bar near
an open-circuit anode to atmospheric or even pressurised conditions on the
cathode side, and with sufficient electrical conductivity. Lanthanum chromite
is electricaIly conductive and stable, but unfortunately has negligible
electrocatalytic activity. Baker and Metcalfe [47] therefore applied the
strategy of substitution, with calcium on the A site and with nickel or cobalt on
the B site. Primdahl et al. [48] used 3% vanadium on the chromium site of a
lanthanum-strontium perovskite. Use of a lower valency metal ion as substituent
on the A site requires compensation, either by a higher valency ion on the B site
or by oxygen lattice defects, which can increase the activity of the material
towards oxygen exchange and catalysis in comparison with the non-catalytic
parent structure. Sauvet et aL, in a review of oxide-based anodes [49], noted that
to enhance the activity of a chromite substituted partialIy with strontium on the
A site, a C-H bond breaking catalyst, specifically nickel, ruthenium or platinum,
is required on the B site. The catalytic oxidation of methane over ceria and
chromites promotes deep oxidation, producing C02 and water. However nickel
substituting up to 10% of the chromium sites gives selectivity for hydrogen and
CO in the temperature range 500-800°C [SO]. In fuel cell operation the low
surface coverage of metallic nickel avoids carbon deposition while providing the
selective sites necessary for fuel activation [S 13. Finally, mention should be made
of exotic options, like the vanadium carbide anode for oxidation of gas-entrained
solid fuel [ 5 21.

6.10 Summary

Empirical development of the nickel-zirconia anode over several decades has led
to solid oxide fuel cells with adequate service life and performance, but fuel
reforming is still required to operate with commercially available hydrocarbon
fuels. It has become evident that the anode reactions are dominated by the
‘three-phase boundary’ and that the microstructure of the composite cermet
anodes is pivotal. Consequently, the processing methods used for making
the anode powders, and the fabrication techniques used for deposition on the
eIectrolyte are critical in making high performance anodes. Anode-supported
cells with very thin electrolyte films are becoming interesting for operation at
lower temperatures.
Anode behaviour is evaluated by d.c. methods under steady state and by
impedance spectroscopy under transient conditions. The reaction pathways
for hydrogen have been elucidated, and mathematical modelling is providing
micro- and nanoscale understanding of electrode processes. At higher current
loadings, the diffusion processes have been evaluated showing that the
electrochemically active anode thickness is around 10 pm. In practice, however,